Skeletal muscle atrophy is a common disorder caused by aging, illness, muscle disuse and fasting. Frequent and serious effects of skeletal muscle atrophy include weakness, falls, fractures, prolonged hospitalization, insulin resistance, loss of independent living and impaired quality of life. Despite these broad implications for human health, skeletal muscle atrophy remains poorly understood and lacks a pharmacological therapy. In preliminary studies, we identified a molecular signaling pathway that promotes muscle atrophy. In human and mouse skeletal muscle, diverse stresses including aging, illness, muscle disuse and fasting increase the level of Gadd45a protein. Gadd45a then translocates to nuclei of skeletal muscle fibers and stimulates demethylation of the Cdkn1a gene promoter. This activates the Cdkn1a gene and increases the level of Cdkn1a protein. Importantly, Cdkn1a increases protein breakdown, decreases protein synthesis, and ultimately, causes skeletal muscle atrophy. Taken together, these preliminary data reveal a new role for active DNA demethylation in muscle atrophy, and identify Gadd45a-stimulated Cdkn1a demethylation as a critical molecular event. However, the mechanisms that control Cdkn1a methylation in skeletal muscle remain poorly understood. These mechanisms represent potential therapeutic targets for medicines to prevent and treat muscle atrophy in humans who are ill or aged. In the proposed studies, we will use mouse and cultured myotube models to determine molecular mechanisms that control Cdkn1a methylation in skeletal muscle. In Specific Aim 1, we will test the hypothesis that nucleotide excision repair proteins interact with Gadd45a to form multimeric complexes that actively demethylate Cdkn1a and cause muscle atrophy. In Specific Aim 2, we will test the hypothesis that DNA methyltransferase 3a mediates Cdkn1a re-methylation, thereby stimulating the recovery of muscle mass. Through these studies, we hope to discover novel molecular mechanisms that control skeletal muscle mass, as well as new potential therapeutic targets for a disabling condition that affects millions of patients and elderly individuals.